Hi All:
I'm quite sure the 120s were made at the same time as the double threaded 143s. Since there are so few of either around, I can't imagine the double threading trial would be tried twice.
For some reason, I recall it being said that a cd120 was found in a house or on a house once. I know one was found on a side pin in a garage sale, but this is not 100% proof it was used this way. The insulator was a reasonable fit if I recall. The pin must have been a bit smaller in diameter or something. I forget the exact details.
I'm leaning towards residential use rather than crossarm installations for the 120 double threaded insulators.
Continued good luck with those double thread 143s Andy!
Robin